Paliperidone is chemically known as 3-[2-[4-(6-Fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]ethyl]-6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one and represented as follows:

U.S. Pat. No. 5,158,952 describes various processes for the synthesis of paliperidone in which one route involves condensation of 3-(2-chloroethyl)-6,7,8,9-tetrahydro-9-hydroxy-2-methyl-4H-pyrido[1,2-a]pyrimidin-4-one with 6-fluoro-3-(4-piperidinyl)-1,2-benzisoxazole hydrochloride in the presence of a base followed by purification using column chromatography and recrystallization in a suitable solvent. Disadvantages of this process are low yields and purification by column chromatography.
It is known that a drug substance prepared by chemical synthesis usually contains impurities resulting from its synthesis or degradation. These impurities include unreacted starting materials, reagents, by-products of the reaction, products of the side reactions or degradation products.
9-oxo-risperidone (I) is one of the impurities which is formed during the synthesis of paliperidone and which affects the purity of the final product.
This impurity is also known as PLP-ceto or keto impurity. The structure of this impurity is as follows:

The undesired impurities reduce the purity of the Active Pharmaceutical Ingredient (API) in the pharmaceutical product and often decrease the stability and shelf life of the pharmaceutical product. Thus, reducing or limiting these undesired impurities becomes very crucial.
The normal techniques of reducing such impurities are known to the skilled person, such as fractional crystallization or preferably by optimizing reaction conditions whereby impurity formation is reduced or avoided. Alternatives, in cases where a product is obtained having related impurities generated in the process of preparation, is to resort to expensive column chromatography or the preparation of derivatives such as salts, esters in order to purify and regenerate the final product in a pure form.
WO2008140641 describes purification of paliperidone by crystallization or by slurrying paliperidone in an organic solvent.
WO2011006638 describes preparation of paliperidone in an organic solvent and in the presence of water wherein the pH of the reaction medium is between 6 and 14. Further, purification of paliperidone is achieved by crystallization in the presence of active charcoal having neutral pH value and the content of individual metal cations less than 200 ppm.
Several attempts have been made to reduce the level of keto impurity, and various patent applications have been filed which describe purification methods to reduce the level of keto impurity in paliperidone. However, these methods are lengthy and tedious, requiring dissolution in a solvent, refluxing for several hours, further isolation and crystallization. The methods are also costly as they involve the use of additional solvents.
WO2009118655 describes purification of crude paliperidone by contacting it with reducing agent. The reducing agents include metal hydrides, specifically sodium borohydride. However, this application involves purification of crude paliperidone by a series of dissolution, filtration and washing steps. The process also requires treatment with activated carbon or silica gel.
WO2010003703 describes preparation, isolation of keto impurity and its further use in the preparation of paliperidone. Keto impurity is converted into paliperidone by reduction using hydride, specifically sodium borohydride.
WO2010004578 discloses various processes for preparation of paliperidone. On page 10, step b), it describes that the paliperidone obtained in the earlier step is treated with sodium borohydride to convert any amount of keto impurity that is formed and present as an impurity, into paliperidone. The process disclosed on page 11 describes that keto impurity is first prepared and then treated with suitable reducing agent, which is sodium borohydride.
Both processes require further stirring of 1 hour after addition of reducing agent, and then an additional one and a half hours for purification and isolation, which is again time consuming.
All the processes described in these prior art applications involve preparation and isolation of paliperidone and/or keto impurity and then treatment with reducing agent, or treating paliperidone and/or keto impurity formed in the reaction with reducing agent, which makes the process lengthy, costly and also affects the yield of the final product.
Since it is difficult to remove the impurities by the above techniques, and the methods known in the art are cumbersome and involve additional steps causing yield loss, there is a need to provide an improved process that is industrially favorable, provides substantially pure paliperidone or a pharmaceutically acceptable salt thereof, and which also provides a better yield of paliperidone.